Sludge treatment method

ABSTRACT

A sludge treatment method allowing an improvement in a phosphorus recovery rate and sludge volume reduction by efficiently eluting phosphorus in the sludge. The sludge treatment method includes: a step of foaming a sludge-containing liquid by blowing an ozone-containing gas into the sludge-containing liquid; and a step of eluting phosphorus in the sludge-containing liquid by bringing sludge adsorbed on bubbles and a sludge-dissolving agent into contact with each other. The sludge treatment method may further include the steps of: separating the phosphorus eluted sludge-containing liquid into a phosphorus eluate and a residual sludge; and precipitating a phosphorus compound by adding a coagulant to the separated phosphorus eluate.

CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATED BY REFERENCE

This application is a divisional of and claims the benefit of priorityunder 35 USC §120 from U.S. Ser. No. 10/961,113, filed Oct. 12, 2004 andis based upon and claims the benefit of priority under 35 USC §119 fromthe Japanese Patent Applications No. 2004-402136 filed Jan. 7, 2004, and2004-160048, filed May 28, 2004, the entire contents of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a sludge treatment method and a sludgetreatment apparatus for treating sludge generated during treatment ofsewage, food wastewater, livestock wastewater, or the like.

BACKGROUND OF THE INVENTION

It is estimated that phosphate rock, which is a raw material forphosphorus, will be depleted in less than 100 years. Japan imports 100%of its phosphate rock, and phosphorus is indispensable in agriculturalproduction. Thus, food shortages may become a serious problem if nothingis done. Against such a background, attempts have been made to recycleorganic waste by recovering and recycling phosphorus from activatedsludge used in sewage treatment, food wastewater treatment, livestockwastewater treatment, or the like. In particular, the phosphorus contentin sludge has increased recently through advances in sewage treatment,and thus, reduction of environmental load by recovering phosphorus fromsewage sludge has become important.

An example of a known sludge treatment method including phosphorusrecovery involves: solubilizing sludge using ozone or ultrasonic waves;separating solids from liquids; and adding Ca or Mg ions to theseparated liquid to precipitate the barely soluble phosphate, to therebyrecover phosphorus (see JP 2003-047988 A, for example). Further, anexample of a known sludge treatment method employing foaming involves:blowing an ozone-containing gas through sludge to form a gas-liquidcontact region; and subjecting the sludge to ozone treatment whilemaintaining the gas-liquid contact region at a constant height (see JP08-267099 A, for example). However, both the methods have problems inthat phosphorus in the sludge can not be sufficiently eluted and thephosphorus recovery rate is low.

Thus, a method of eluting phosphorus in sludge more efficiently thanconventional methods has been proposed, which involves: subjecting thesludge to ozone treatment; and then treating the sludge with an alkalisuch as sodium hydroxide (see JP 2003-200193 A, for example).

However, in conventional methods involving alkali treatment after ozonetreatment, ozone is not efficiently used for reaction with the sludgeeven if a large volume of ozone-containing gas is blown because theozone is wasted through a reaction with dissolved components. Thus,conventional methods have problems in that the phosphorus recovery rateis about 70%, which is far from being sufficient, and in that treatmentcannot be carried out efficiently. Further, the methods have problems inthat the residence time of the sludge had to be long for sufficientreaction to occur between the sludge and the ozone to reduce sludgevolume because of the small gas-liquid contact area.

SUMMARY OF THE INVENTION

The present invention has been made in view of solving the aboveproblems, and an object of the present invention is therefore to providea sludge treatment method and a sludge treatment apparatus that allowimprovements in phosphorus recovery rates and sludge volume reduction byefficiently eluting phosphorus in the sludge.

The inventors of the present invention, through intensive studies onsludge treatment methods using ozone, have found that ozone and sludgecan react directly with each other and that ozone can be effectivelyused, by adsorbing the sludge on bubbles. Further, the inventors of thepresent invention have found that phosphorus can be remarkably eluted bybringing the sludge adsorbed on bubbles and a sludge-dissolving agent incontact with each other, to complete the invention.

That is, the present invention relates to a sludge treatment methodincluding the steps of: foaming a sludge-containing liquid by blowing anozone-containing gas into the sludge-containing liquid; and elutingphosphorus in the sludge-containing liquid by bringing sludge adsorbedon bubbles and a sludge-dissolving agent into contact with each other.

Further, the present invention provides a sludge treatment apparatusincluding: an ozone treatment vessel which receives a suppliedsludge-containing liquid and foams the sludge-containing liquid byblowing an ozone-containing gas into the sludge-containing liquid; and aphosphorus elution vessel which receives bubbles generated in the ozonetreatment vessel and elutes phosphorus in the sludge-containing liquidby bringing sludge adsorbed on the bubbles and a sludge-dissolving agentinto contact with each other.

According to the present invention, an improved phosphorus recovery rateand sludge volume reduction can be achieved by: foaming asludge-containing liquid by blowing an ozone-containing gas into thesludge-containing liquid; and efficiently eluting phosphorus in thesludge by bringing the sludge adsorbed on bubbles and asludge-dissolving agent into contact with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 1 of the present invention;

FIG. 2 is a diagram illustrating an ozone treatment vessel of the sludgetreatment apparatus according to Embodiment 1 of the present invention;

FIG. 3 is a diagram illustrating a phosphorus elution vessel of thesludge treatment apparatus according to Embodiment 1 of the presentinvention;

FIG. 4 is a graph showing a phosphorus elution rate in Embodiment 1;

FIG. 5 is a graph showing a VSS reduction rate in Embodiment 1;

FIG. 6 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 2 of the present invention;

FIG. 7 is a diagram illustrating a phosphorus elution vessel of thesludge treatment apparatus according to Embodiment 2 of the presentinvention;

FIG. 8 is a diagram illustrating a phosphorus elution accelerationvessel of the sludge treatment apparatus according to Embodiment 2 ofthe present invention;

FIG. 9 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 3 of the present invention;

FIG. 10 is a diagram illustrating an ozone treatment vessel of thesludge treatment apparatus according to Embodiment 3 of the presentinvention;

FIG. 11 is a diagram illustrating a phosphorus elution vessel of thesludge treatment apparatus according to Embodiment 3 of the presentinvention;

FIG. 12 is a diagram illustrating a phosphorus elution accelerationvessel of the sludge treatment apparatus according to Embodiment 3 ofthe present invention;

FIG. 13 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 4 of the present invention;

FIG. 14 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 5 of the present invention;

FIG. 15 is a diagram illustrating a phosphorus elution vessel of thesludge treatment apparatus according to Embodiment 5 of the presentinvention;

FIG. 16 is a diagram illustrating a phosphorus elution accelerationvessel of the sludge treatment apparatus according to Embodiment 5 ofthe present invention; and

FIG. 17 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 6 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

FIG. 1 is a block diagram showing a construction of a sludge treatmentapparatus according to Embodiment 1 of the present invention.

In FIG. 1, a sludge treatment apparatus 10 is provided with an ozonetreatment vessel 11 and a phosphorus elution vessel 12, and thephosphorus elution vessel 12 is connected downstream of the ozonetreatment vessel 11. An ozonizer 13, which generates an ozone-containinggas d, is connected to the ozone treatment vessel 11. Asludge-dissolving agent storage vessel 15, which temporarily stores asludge-dissolving agent e, is connected to the phosphorus elution vessel12 through a sludge-dissolving agent supply pump 14. A waste ozone gasdecomposition device 16, which decomposes ozone gas not dissolved in asludge-containing liquid a into oxygen and discharges oxygen ozone intothe atmosphere, is connected to an upper portion of the phosphoruselution vessel 12. A discharge pipe 17, which discharges a phosphoruseluted sludge-containing liquid b, is connected to a lower portion ormiddle portion of the phosphorus elution vessel 12.

Sludge treatment using such a sludge treatment apparatus 10 firstinvolves: supplying the sludge-containing liquid a generated from asewage treatment plant (not shown) or the like to the ozone treatmentvessel 11; and blowing the ozone-containing gas d generated from theozonizer 13 into the sludge-containing liquid a in the ozone treatmentvessel 11. Oxidative decomposition of the sludge proceeds by virtue ofthe oxidative power of ozone, thereby foaming the sludge-containingliquid a. Bubbles are readily generated in the ozone treatment vessel11, to thereby adsorb the sludge on the bubbles. However, the amount ofphosphorus eluting from the sludge into a liquid phase is small at thispoint, and the phosphorus elution rate is about several % at most.Sludge adsorbed bubbles c and a residual liquid h containing sludge thatremains without being adsorbed on the bubbles are supplied to thephosphorus elution vessel 12 through a bubble discharge port 22. At thesame time, the sludge-dissolving agent e including an alkaline aqueoussolution such as an aqueous solution of sodium hydroxide or an aqueoussolution of potassium hydroxide, and an acidic aqueous solution such ashydrochloric acid or sulfuric acid is supplied from thesludge-dissolving agent storage vessel 15 to the phosphorus elutionvessel 12 through the sludge-dissolving agent supply pump 14. At thistime in the phosphorus elution vessel 12, the sludge adsorbed bubbles care eliminated, while the sludge which is in an easily phosphorus-elutedstate through ozone treatment and the sludge-dissolving agent e arebrought into contact with each other for a reaction, thereby eluting alarge amount of phosphorus in the sludge into a liquid phase. Thephosphorus eluted sludge-containing liquid b is discharged to theoutside of the vessel through the discharge pipe 17. Of theozone-containing gas d supplied to the ozone treatment vessel 11, ozonewhich could not dissolve in the sludge-containing liquid a is decomposedinto oxygen and discharged to the atmosphere by the waste ozone gasdecomposition device 16 connected to the upper portion of the phosphoruselution vessel 12.

Next, a step of foaming the sludge-containing liquid a by blowing theozone-containing gas d into the sludge-containing liquid a in the ozonetreatment vessel 11 will be described specifically with reference toFIG. 2.

The ozone treatment vessel 11 is provided with a diffuser pipe 18, amixer 19, a bubble mixing blade 20, and a liquid mixing blade 21. Abubble discharge port 22 is provided in an upper portion of the ozonetreatment vessel 11. The shape of the ozone treatment vessel 11 is notparticularly limited as long as the vessel allows efficient dissolutionof ozone in the sludge, but is preferably of a diffuser-type or anejector type. First, the sludge-containing liquid a is supplied to theozone treatment vessel 11 at a predetermined flow rate. Theozone-containing gas d is supplied from the ozonizer 13 into thesludge-containing liquid a through the diffuser pipe 18, while apredetermined volume of the sludge-containing liquid a is stored in theozone treatment vessel 11. Thus, the sludge and ozone react with eachother, thereby foaming the sludge-containing liquid a. Through thisreaction, strong cell walls of the sludge (microorganisms) are destroyedwhile carbonate groups (carbonate ions or bicarbonate ions) are removed.The mixer 19 is operated to rotate the bubble mixing blade 20 and theliquid mixing blade 21 provided at predetermined positions. Thus, thesludge adsorbed bubbles c and the residual liquid h containing thesludge remaining without being adsorbed on the bubbles are stirred,thereby accelerating the reaction between the sludge and ozone. Thesludge adsorbed bubbles c and the residual liquid h rise by virtue ofpressure of the ozone-containing gas d, and are supplied to thephosphorus elution vessel 12 through the bubble discharge port 22.

Here, the supply of the ozone-containing gas d is preferably 10 to 500mg·O₃/g·SS, more preferably 20 to 400 mg·O₃/g·SS. A supply of theozone-containing gas d of less than 10 mg·O₃/g·SS may reduce thephosphorus elution rate from the sludge because the microorganisms inthe sludge cannot be sufficiently damaged. A supply of theozone-containing gas d exceeding 500 mg·O₃/g·SS may result in hightreatment cost because ozone is wasted.

Further, a ratio (G/L) of the ozone-containing gas supply to the sludgesupply is preferably 0.05 or more. A ratio G/L of less than 0.05 mayreduce the rate of oxidative decomposition reaction of the sludgebecause the bubbles barely generate.

The residence time of the sludge-containing liquid a in the ozonetreatment vessel 11 is preferably 30 minutes or less. A residence timeexceeding 30 minutes requires a large ozone treatment vessel 11, whichmay lead to a large sludge treatment apparatus 10.

Next, the step of eluting phosphorus in the sludge into the liquid phaseby bringing the sludge adsorbed bubbles c and the residual liquid h, andthe sludge-dissolving agent into contact with each other in thephosphorus elution vessel 12 will be described specifically withreference to FIG. 3.

The phosphorus elution vessel 12 is provided with the mixer 19, theliquid mixing blade 21, and a defoaming blade 23. The sludge adsorbedbubbles c and the residual liquid h are supplied from the ozonetreatment vessel 11 to the phosphorus elution vessel 12, while thesludge-dissolving agent e is supplied from the sludge-dissolving agentstorage vessel 15 to the phosphorus elution vessel 12. The mixer 19 isoperated to eliminate bubbles with the defoaming blade 23 and to mix thephosphorus eluted sludge-containing liquid b and the sludge-dissolvingagent e with the liquid mixing blade 21, thereby accelerating phosphoruselution. The number of blades on the defoaming blade 23 is preferablylarger than the number of blades on the liquid mixing blade 21 forefficient defoaming. The phosphorus eluted sludge-containing liquid b isdischarged to the outside of the vessel through the discharge pipe 17.The liquid level in the phosphorus elution vessel 12 can be adjusted toa constant height by providing an overflow type discharge pipe 17 or byproviding a liquid level sensor in the vessel to discharge thephosphorus eluted sludge-containing liquid b using a pump interlockedwith the sensor. Of the ozone gas supplied to the ozone treatment vessel11, any ozone gas which could not dissolve in the sludge-containingliquid a is decomposed into oxygen and discharged to the atmosphere bythe waste ozone gas decomposition device 16 connected to the upperportion of the phosphorus elution vessel 12.

The residence time of the phosphorus eluted sludge-containing liquid bin the phosphorus elution vessel 12 is preferably 30 minutes or less. Aresidence time exceeding 30 minutes requires a large phosphorus elutionvessel 12, which may lead to a large sludge treatment apparatus 26.

The method for supplying the sludge-dissolving agent e here is notparticularly limited. The sludge-dissolving agent e may be supplied as ashower from the upper portion of the phosphorus elution vessel 12 or maybe directly supplied just before the sludge adsorbed bubbles c and theresidual liquid h are supplied to the phosphorus elution vessel 12.

The sludge in the phosphorus elution vessel 12 has a pH of preferably 10or more, more preferably 13 or more, when an alkaline aqueous solutionis used as the sludge-dissolving agent e. A pH of less than 10 mayreduce the phosphorus elution rate because the dissolution of the sludgewill not proceed. An example of a method of adjusting the pH of thesludge to the above values involves: providing a pH meter for measuringthe pH of the sludge in the phosphorus elution vessel 12; and supplyingthe sludge-dissolving agent e in accordance with the pH measured.Alternatively, the volume of the alkaline aqueous solution added withrespect to the sludge volume may be adjusted to a constant value insteadof adjusting the pH of the sludge. In this case, the sludge preferablyhas a hydroxide ion concentration of 0.1 mmol/L or more.

The sludge in the phosphorus elution vessel 12 has a pH of preferably 4or less, more preferably 3 or less, when an acidic aqueous solution isused as the sludge-dissolving agent e. A pH of the sludge exceeding 4may reduce the phosphorus elution rate because the dissolution of thesludge will not proceed.

A sludge treatment experiment was carried out using such a sludgetreatment apparatus 10 under the conditions of SS (Suspended solids) inthe sludge of 4,300 mg/L, an ozone-containing gas supply of 180mg·O₃/g·SS, a pH of the sludge in the phosphorus elution vessel of 13(adjusted with 8 mol/L of sodium hydroxide solution), and treatment timeof 30 minutes, to thereby determine a phosphorus elution rate from thesludge. FIG. 4 shows the results. Further, a VSS (Volatile suspendedsolids) reduction rate was determined under similar conditions. FIG. 5shows the results.

A sludge treatment experiment was carried out using a conventionalapparatus which involves alkali treatment after the ozone treatmentunder similar conditions, to thereby determine the phosphorus elutionrate from the sludge. FIG. 4 shows the results. Further, the VSSreduction rate was determined under similar conditions, and FIG. 5 showsthe results.

FIG. 4 clearly shows that the phosphorus elution rate remained at 67.6%with the conventional apparatus, while the phosphorus elution ratesignificantly improved to 89.0% with the sludge treatment apparatus 10.Further, FIG. 5 clearly shows that the VSS reduction rate from thesludge remained at 31% with the conventional apparatus, while the VSSreduction rate from the sludge significantly improved to 55.0% with thesludge treatment apparatus 10. The sludge treatment apparatus 10 ofEmbodiment 1 of the present invention has a large contact area betweenthe sludge and ozone. Thus, the sludge treatment apparatus 10 allowsefficient elution of phosphorus in the sludge into the liquid phase andsignificant sludge volume reduction during sludge treatment. Inaddition, such a phosphorus elution rate and a VSS reduction rate couldnot be attained in treatment lasting for a short time of 30 minutesthrough other conventional phosphorus elution treatments such as heattreatment or ultrasonic disintegration treatment.

Embodiment 2

FIG. 6 is a block diagram showing a construction of a sludge treatmentapparatus 26 according to Embodiment 2 of the present invention.

In FIG. 6, the sludge treatment apparatus 26 is provided with an ozonetreatment vessel 11, a phosphorus elution vessel 12A, and a phosphoruselution acceleration vessel 27. The phosphorus elution vessel 12A isconnected downstream of the ozone treatment vessel 11, and thephosphorus elution acceleration vessel 27 is connected downstream of thephosphorus elution vessel 12A. A waste ozone gas decomposition device 16is connected to an upper portion of both the phosphorus elution vessel12A and the phosphorus elution acceleration vessel 27. Discharge pipes17A and 17B, which discharge a phosphorus eluted sludge-containingliquid b, are provided in a bottom portion of the phosphorus elutionvessel 12A and in a lower portion or middle portion of the phosphoruselution acceleration vessel 27. The other construction is the same asthe construction shown in FIG. 1, and thus, the same portions as thosein FIG. 1 are represented by the same reference, numerals anddescriptions thereof are omitted in Embodiment 2 of the presentinvention.

Sludge treatment using such a sludge treatment apparatus 26 firstinvolves supplying a sludge-containing liquid a generated from a sewagetreatment plant (not shown) or the like to the ozone treatment vessel11. Through blowing of ozone, sludge adsorbed bubbles c and a residualliquid h containing sludge that remains without being adsorbed on thebubbles are supplied to the phosphorus elution vessel 12A through abubble discharge port 22. At the same time, a sludge-dissolving agent eincluding an alkaline aqueous solution such as an aqueous solution ofsodium hydroxide or an aqueous solution of potassium hydroxide, and anacidic aqueous solution such as hydrochloric acid or sulfuric acid issupplied from a sludge-dissolving agent storage vessel 15 to thephosphorus elution vessel 12A through a sludge-dissolving agent supplypump 14. At this time in the phosphorus elution vessel 12A, the sludgeadsorbed bubbles c are eliminated, while the sludge which is in aneasily phosphorus-eluted state through ozone treatment and thesludge-dissolving agent e are brought into contact with each other for areaction, thereby eluting a large amount of phosphorus in the sludgeinto a liquid phase. The phosphorus eluted sludge-containing liquid b issupplied to the phosphorus elution acceleration vessel 27 through thedischarge pipe 17A. In the phosphorus elution acceleration vessel 27,the phosphorus eluted sludge-containing liquid b is further eluted intothe liquid phase through mixing.

Next, the phosphorus elution vessel 12A will be described specificallywith reference to FIG. 7.

The phosphorus elution vessel 12A is provided with a mixer 19, adefoaming blade 23, a bottom portion mixing blade 24, and a bladeappendant 25. The sludge adsorbed bubbles c and the residual liquid hare supplied from the ozone treatment vessel 11 to the phosphoruselution vessel 12A, while the sludge-dissolving agent e is supplied fromthe sludge-dissolving agent storage vessel 15 to the phosphorus elutionvessel 12A. The sludge which is in an easily phosphorus-eluted state andthe sludge-dissolving agent e react with each other, to thereby elute alarge amount of phosphorus in the sludge into the liquid phase. Themixer 19 is operated to eliminate bubbles with the defoaming blade 23.The bubbles remaining are eliminated by coming into contact with theblade appendant 25 made of silicone tubes or the like. Through thisreaction, strong cell walls of the sludge (microorganisms) are furtherfragmented, and the sludge is modified (barely biodegradable substancesbecome easily biodegradable). The phosphorus eluted sludge-containingliquid b falls to a bottom portion of the phosphorus elution vessel 12A,is further stirred by the bottom portion mixing blade 24, and isdischarged to the outside of the vessel through the discharge pipe 17A.Here, the bottom portion of the phosphorus elution vessel 12A preferablyhas an angle of inclination α of 10° or more, so that the sludge mayfall smoothly. Of the ozone gas supplied to the ozone treatment vessel11, any ozone gas which could not dissolve in the sludge-containingliquid a is decomposed into oxygen and discharged to the atmosphere bythe waste ozone gas decomposition device 16 connected to the upperportion of the phosphorus elution vessel 12.

Next, the phosphorus elution acceleration vessel 27 will be describedspecifically with reference to FIG. 8.

The phosphorus elution acceleration vessel 27 is provided with a mixer19, a liquid mixing blade 21, and a defoaming blade 23. The phosphoruseluted sludge-containing liquid b is supplied from the phosphoruselution vessel 12A to the phosphorus elution acceleration vessel 27. Themixer 19 is operated to eliminate bubbles partially remaining in thephosphorus elution vessel 12A or bubbles generated through mixing withthe defoaming blade 23, and to further mix the phosphorus elutedsludge-containing liquid b with the liquid mixing blade 21, therebyaccelerating phosphorus elution. The phosphorus eluted sludge-containingliquid b is discharged to the outside of the vessel through thedischarge pipe 17B. The liquid level in the phosphorus elutionacceleration vessel 27 here can be adjusted to a constant height byproviding an overflow type discharge pipe 17B or by providing a liquidlevel sensor in the vessel to discharge the phosphorus elutedsludge-containing liquid b using a pump interlocked with the sensor. Anyozone gas which could not dissolve in the sludge-containing liquid a isdecomposed into oxygen and discharged to the atmosphere by the wasteozone gas decomposition device 16 connected to the upper portion of thephosphorus elution acceleration vessel 27.

The residence time of the phosphorus eluted sludge-containing liquid bin the phosphorus elution acceleration vessel 27 is preferably 30minutes or less. A residence time exceeding 30 minutes requires a largephosphorus elution acceleration vessel 27, which may lead to a largesludge treatment apparatus 26.

According to Embodiment 2 of the present invention, a long contact timeis provided between the sludge and the sludge-dissolving agent e,thereby further improving the phosphorus elution rate. Further, thesludge and the bubbles are completely separated, and thus, no bubblesare delivered into the waste ozone gas decomposition device 16, therebypreventing contamination of the waste ozone gas decomposition device 16.

In Embodiment 2 of the present invention, the sludge-dissolving agent eis supplied to the phosphorus elution vessel 12A, but thesludge-dissolving agent e may be supplied to both the phosphorus elutionvessel 12A and the phosphorus elution acceleration vessel 27. The methodfor supplying the sludge-dissolving agent e is not particularly limited.The sludge-dissolving agent e may be supplied as a shower from the upperportion of both of the vessels (phosphorus elution vessel 12A andphosphorus elution acceleration vessel 27). Alternatively, thesludge-dissolving agent e may be directly supplied just before thesludge adsorbed bubbles c and the residual liquid h are supplied, or thephosphorus eluted sludge-containing liquid b is supplied to both of thevessels.

Embodiment 3

FIG. 9 is a block diagram showing a construction of a sludge treatmentapparatus 29 according to Embodiment 3 of the present invention.

In FIG. 9, the sludge treatment apparatus 29 is provided with an ozonetreatment vessel 11A, a phosphorus elution vessel 12B, and a phosphoruselution acceleration vessel 27A. As shown in FIG. 10, an overflow typeresidual liquid discharge port is provided in a lower portion or middleportion of the ozone treatment vessel 11A, and the residual liquiddischarge port of the ozone treatment vessel 11A is connected to thephosphorus elution acceleration vessel 27A through a residual liquiddischarge pipe 28. The other construction is the same as theconstruction shown in FIG. 6, and thus, the same portions as those inFIG. 6 are represented by the same reference, numerals and descriptionsthereof are omitted in Embodiment 3 of the present invention.

Sludge treatment using such a sludge treatment apparatus 29 firstinvolves supplying a sludge-containing liquid a generated from a sewagetreatment plant (not shown) or the like to the ozone treatment vessel11A. Through blowing of ozone, sludge adsorbed bubbles c are supplied tothe phosphorus elution vessel 12B through a bubble discharge port 22. Atthe same time, a sludge-dissolving agent e including an alkaline aqueoussolution such as an aqueous solution of sodium hydroxide or an aqueoussolution of potassium hydroxide, and an acidic aqueous solution such ashydrochloric acid or sulfuric acid is supplied from a sludge-dissolvingagent storage vessel 15 to the phosphorus elution vessel 12B through asludge-dissolving agent supply pump 14. At this time in the phosphoruselution vessel 12B, the sludge adsorbed bubbles c are eliminated and thesludge which is in an easily phosphorus-eluted state through ozonetreatment and the sludge-dissolving agent e are brought into contactwith each other for a reaction, thereby eluting a large amount ofphosphorus in the sludge into the liquid phase. A phosphorus elutedsludge-containing liquid b is supplied from the phosphorus elutionvessel 12B to the phosphorus elution acceleration vessel 27A through adischarge pipe 17A, while a residual liquid h containing sludge thatremains without being adsorbed on the bubbles is directly supplied fromthe ozone treatment vessel 11A to the phosphorus elution accelerationvessel 27A through the residual liquid discharge pipe 28. Thus, in thephosphorus elution acceleration vessel 27A, the phosphorus elutedsludge-containing liquid b and the residual liquid h are mixed.

A boundary between the sludge adsorbed bubbles c and the residual liquidh can be maintained constant by adjusting a pressure balance between thesludge adsorbed bubbles c and the residual liquid h, that is, by:adjusting a liquid level in the phosphorus elution acceleration vessel27A (adjusting to a position higher than the boundary between the sludgeadsorbed bubbles c and the residual liquid h), when the liquid level inthe phosphorus elution acceleration vessel 27A is higher than thehighest part of the residual liquid discharge pipe 28; and adjusting theheight of the highest part of the residual liquid discharge pipe 28(adjusting to a position higher than the boundary between the sludgeadsorbed bubbles c and the residual liquid h), when the highest part ofthe residual liquid discharge pipe 28 is higher than the liquid level inthe phosphorus elution acceleration vessel 27A.

Most of the sludge is adsorbed on the bubbles in the ozone treatmentvessel 11A, and thus, most phosphorus in the sludge-containing liquid aelutes in the phosphorus elution vessel 12B. Thus, the residual liquid hcontains only about several % to several tens % of SS in thesludge-containing liquid a, and the residual liquid contains hardly anyphosphorus.

Next, the phosphorus elution vessel 12B will be described specificallywith reference to FIG. 11.

The phosphorus elution vessel 12B is provided with a mixer 19, adefoaming blade 23, a bottom portion mixing blade 24, and a bladeappendant 25. The sludge adsorbed bubbles c and the residual liquid hare supplied from the ozone treatment vessel 11A to the phosphoruselution vessel 12B, while the sludge-dissolving agent e is supplied fromthe sludge-dissolving agent storage vessel 15 to the phosphorus elutionvessel 12B. The sludge which is in an easily phosphorus-eluted state andthe sludge-dissolving agent e react with each other, thereby eluting alarge amount of phosphorus in the sludge into a liquid phase. The mixer19 is operated to eliminate bubbles with the defoaming blade 23. Thebubbles remaining are eliminated by coming into contact with the bladeappendant 25 made of silicone tubes or the like. Through this reaction,strong cell walls of the sludge (microorganisms) are further fragmented,and the sludge is modified (barely biodegradable substances becomeeasily biodegradable). The phosphorus eluted sludge-containing liquid bfalls to a bottom portion of the phosphorus elution vessel 12B, isfurther stirred by the bottom portion mixing blade 24, and is dischargedto the outside of the vessel through the discharge pipe 17A. Here, thebottom portion of the phosphorus elution vessel 12B preferably has anangle of inclination α of 10° or more, so that the sludge may fallsmoothly. Of the ozone gas supplied to the ozone treatment vessel 11A,any ozone gas which could not dissolve in the sludge-containing liquid ais decomposed into oxygen and discharged to the atmosphere by the wasteozone gas decomposition device 16 connected to the upper portion of thephosphorus elution vessel 12B.

Next, the phosphorus elution acceleration vessel 27A will be describedspecifically with reference to FIG. 12.

The phosphorus elution acceleration vessel 27A is provided with a mixer19, a liquid mixing blade 21, and a defoaming blade 23. The phosphoruseluted sludge-containing liquid b is supplied from the phosphoruselution vessel 12B to the phosphorus elution acceleration vessel 27A,while the residual liquid h is directly supplied from the ozonetreatment vessel 11A to the phosphorus elution acceleration vessel 27A.The mixer 19 is operated to eliminate bubbles partially remaining in thephosphorus elution vessel 12B or bubbles generated through mixing withthe defoaming blade 23 and to mix the phosphorus elutedsludge-containing liquid b and the residual liquid h with the liquidmixing blade 21, thereby accelerating phosphorus elution. The phosphoruseluted sludge-containing liquid b is discharged to the outside of thevessel through the discharge pipe 17B. The liquid level in thephosphorus elution acceleration vessel 27A can be adjusted to a constantheight by providing an overflow type discharge pipe 17B or by providinga liquid level sensor in the vessel to discharge the phosphorus elutedsludge-containing liquid b using a pump interlocked with the sensor. Anyozone gas which could not dissolve in the sludge-containing liquid a isdecomposed into oxygen and discharged to the atmosphere by the wasteozone gas decomposition device 16 connected to the upper portion of thephosphorus elution acceleration vessel 27A.

The residence time of the phosphorus eluted sludge-containing liquid bin the phosphorus elution acceleration vessel 27A is preferably 30minutes or less. A residence time exceeding 30 minutes requires a largephosphorus elution acceleration vessel 27A, which may lead to a largesludge treatment apparatus 29.

According to Embodiment 3 of the present invention, the sludge adsorbedbubbles c and the residual liquid h are separated, thereby allowingefficient elution of phosphorus from the sludge through the addition ofthe sludge-dissolving agent e. That is, the effect of addition of thesludge-dissolving agent e can be enhanced through an efficient reactionbetween the sludge and ozone because ozone is not wasted through areaction with dissolved components in the sludge-containing liquid a.

In Embodiment 3 of the present invention, the sludge-dissolving agent eis supplied to the phosphorus elution vessel 12B, but thesludge-dissolving agent e may be supplied to both the phosphorus elutionvessel 12B and the phosphorus elution acceleration vessel 27A. Themethod for supplying the sludge-dissolving agent e is not particularlylimited. The sludge-dissolving agent e may be supplied as a shower fromthe upper portion of both of the vessels (phosphorus elution vessel 12Band phosphorus elution acceleration vessel 27A). Alternatively, thesludge-dissolving agent e may be directly supplied just before thesludge adsorbed bubbles c and the residual liquid h are supplied or thephosphorus eluted sludge-containing liquid b is supplied to both of thevessels.

Embodiment 4

FIG. 13 is a diagram illustrating a sludge treatment apparatus accordingto Embodiment 4 of the present invention.

In Embodiment 4, a phosphorus elution acceleration vessel 27A isconnected to a phosphorus elution vessel 12B through a circulating pump31 to partially return a phosphorus eluted sludge-containing liquid b inthe phosphorus elution acceleration vessel 27A to the phosphorus elutionvessel 12B. The other construction is the same as the construction shownin FIG. 9, and thus, the same portions as those in FIG. 9 arerepresented by the same reference, numerals and descriptions thereof areomitted in Embodiment 4 of the present invention.

Sludge treatment using such a sludge treatment apparatus 30 firstinvolves supplying a sludge-containing liquid a generated from a sewagetreatment plant (not shown) or the like to the ozone treatment vessel11A. Through blowing of ozone, sludge adsorbed bubbles c are supplied tothe phosphorus elution vessel 12B through a bubble discharge port 22. Atthe same time, a sludge-dissolving agent e including an alkaline aqueoussolution such as an aqueous solution of sodium hydroxide or an aqueoussolution of potassium hydroxide, and an acidic aqueous solution such ashydrochloric acid or sulfuric acid is supplied from a sludge-dissolvingagent storage vessel 15 to the phosphorus elution vessel 12B through asludge-dissolving agent supply pump 14. At this time in the phosphoruselution vessel 12B, the sludge adsorbed bubbles c are eliminated, whilethe sludge which is in an easily phosphorus-eluted state through ozonetreatment and the sludge-dissolving agent e are brought into contactwith each other for a reaction, thereby eluting a large amount ofphosphorus in the sludge into a liquid phase. The phosphorus elutedsludge-containing liquid b is supplied from the phosphorus elutionvessel 12 to the phosphorus elution acceleration vessel 27A through adischarge pipe 17A, while a residual liquid h containing the sludge thatremains without being adsorbed on the bubbles is directly supplied fromthe ozone treatment vessel 11A to the phosphorus elution accelerationvessel 27A through a residual liquid discharge pipe 28. Thus, thephosphorus eluted sludge-containing liquid b and the residual liquid hare mixed in the phosphorus elution acceleration vessel 27A. Thephosphorus eluted sludge-containing liquid b and the residual liquid hare partially returned to the phosphorus elution vessel 12B through thecirculating pump 31, and phosphorus in the sludge is further eluted intothe liquid phase in the phosphorus elution vessel 12B.

A boundary between the sludge adsorbed bubbles c and the residual liquidh in the ozone treatment vessel 11A can be maintained constant byadjusting a pressure balance between the sludge adsorbed bubbles c andthe residual liquid h as in Embodiment 3 of the present invention.

According to Embodiment 4 of the present invention, the phosphoruseluted sludge-containing liquid b and the residual liquid h arepartially returned to the phosphorus elution vessel 12B for repeatedphosphorus elution, thereby further improving the phosphorus elutionrate while maintaining a large sludge treatment volume.

In Embodiment 4 of the present invention, the sludge-dissolving agent ewas supplied to the phosphorus elution vessel 12B, but thesludge-dissolving agent e may be supplied to both the phosphorus elutionvessel 12B and the phosphorus elution acceleration vessel 27A. Themethod for supplying the sludge-dissolving agent e is not particularlylimited. The sludge-dissolving agent e may be supplied as a shower fromthe upper portion of both of the vessels (phosphorus elution vessel 12Band phosphorus elution acceleration vessel 27A). Alternatively, thesludge-dissolving agent e may be directly supplied just before thesludge adsorbed bubbles c and the residual liquid h are supplied or thephosphorus eluted sludge-containing liquid b is supplied to both of thevessels.

Embodiment 5

FIG. 14 is a diagram illustrating a sludge treatment apparatus 32according to Embodiment 5 of the present invention.

In FIG. 14, the sludge treatment apparatus 32 is provided with an ozonetreatment vessel 11A, a phosphorus elution vessel 12C, and a phosphoruselution acceleration vessel 27B. The ozone treatment vessel 11A isconnected to the phosphorus elution vessel 12C through a bubbledischarge port 22 provided in an upper portion of the ozone treatmentvessel 11A. An overflow type residual liquid discharge port provided ina lower portion or middle portion of the ozone treatment vessel 11A isconnected to the phosphorus elution acceleration vessel 27B through aresidual liquid discharge pipe 28. The phosphorus elution vessel 12C andthe phosphorus elution acceleration vessel 27B are each connected to asludge-dissolving agent storage vessel 15, which temporarily stores asludge-dissolving agent e, through a sludge-dissolving agent supply pump14. Discharge pipes 17C and 17D which discharge the phosphorus elutedsludge-containing liquid b are each provided in a lower portion ormiddle portion of the phosphorus elution vessel 12C and the phosphoruselution acceleration vessel 27B. The other construction is the same asthe construction shown in FIG. 9, and thus, the same portions as thosein FIG. 9 are represented by the same reference, numerals anddescriptions thereof are omitted in Embodiment 5 of the presentinvention.

Sludge treatment using such a sludge treatment apparatus 32 firstinvolves supplying a sludge-containing liquid a generated from a sewagetreatment plant (not shown) or the like to the ozone treatment vessel11A. Through blowing of ozone, sludge adsorbed bubbles c are supplied tothe phosphorus elution vessel 12C through the bubble discharge port 22.At the same time, the sludge-dissolving agent e including an alkalineaqueous solution such as an aqueous solution of sodium hydroxide or anaqueous solution of potassium hydroxide, and an acidic aqueous solutionsuch as hydrochloric acid or sulfuric acid is supplied from thesludge-dissolving agent storage vessel 15 to the phosphorus elutionvessel 12C through the sludge-dissolving agent supply pump 14. At thistime in the phosphorus elution vessel 12C, the sludge adsorbed bubbles care eliminated, while the sludge which is in an easily phosphorus-elutedstate through ozone treatment and the sludge-dissolving agent e arebrought into contact with each other for a reaction, thereby eluting alarge amount of phosphorus in the sludge into the liquid phase. Theresidual liquid h containing the sludge that remains without beingadsorbed on the bubbles is directly supplied from the ozone treatmentvessel 11A to the phosphorus elution acceleration vessel 27B through theresidual liquid discharge pipe 28. At the same time, thesludge-dissolving agent e is supplied from the sludge-dissolving agentstorage vessel 15 to the phosphorus elution vessel 12C through thesludge-dissolving agent supply pump 14. At this time, a slight amount ofphosphorus from a small volume of the sludge in the residual liquid helutes into the liquid phase.

The boundary between the sludge adsorbed bubbles c and the residualliquid h in the ozone treatment vessel 11A can be maintained constant byadjusting a pressure balance between the sludge adsorbed bubbles c andthe residual liquid h as in Embodiment 3.

Next, the phosphorus elution vessel 12C will be described specificallywith reference to FIG. 15.

The phosphorus elution vessel 12C is provided with a mixer 19, a liquidmixing blade 21, and a defoaming blade 23. The sludge adsorbed bubbles care supplied from the ozone treatment vessel 11A to the phosphoruselution vessel 12C, while the sludge-dissolving agent e is supplied fromthe sludge-dissolving agent storage vessel 15 to the phosphorus elutionvessel 12C. The mixer 19 is operated to eliminate bubbles with thedefoaming blade 23 and to mix the phosphorus eluted sludge-containingliquid b and the sludge-dissolving agent e with the liquid mixing blade21. The sludge which is in an easily phosphorus-eluted state and thesludge-dissolving agent e react with each other, thereby eluting a largeamount of phosphorus in the sludge into a liquid phase. Through thisreaction, strong cell walls of the sludge (microorganisms) are furtherfragmented, and the sludge is modified (barely biodegradable substancesbecome easily biodegradable). The number of blades on the defoamingblade 23 is preferably larger than the number of blades on the liquidmixing blade 21 for efficient defoaming. The phosphorus elutedsludge-containing liquid b is discharged to the outside of the vesselthrough the discharge pipe 17C. The liquid level in the phosphoruselution vessel 12C can be adjusted to a constant height by providing anoverflow type discharge pipe 17C or by providing a liquid level sensorin the vessel to discharge the phosphorus eluted sludge-containingliquid b using a pump interlocked with the sensor. Any ozone gas whichcould not dissolve in the sludge-containing liquid a is decomposed intooxygen and discharged to the atmosphere by the waste ozone gasdecomposition device 16 connected to the upper portion of the phosphoruselution vessel 12C.

The residence time of the phosphorus eluted sludge-containing liquid bin the phosphorus elution vessel 12C is preferably 30 minutes or less. Aresidence time exceeding 30 minutes requires a large phosphorus elutionvessel 12C, which may lead to a large sludge treatment apparatus 32.

Next, the phosphorus elution acceleration vessel 27B will be describedspecifically with reference to FIG. 16.

The phosphorus elution acceleration vessel 27B is provided with a mixer19, a liquid mixing blade 21, and a defoaming blade 23. The residualliquid h is directly supplied from the ozone treatment vessel 11A to thephosphorus elution acceleration vessel 27B, while the sludge-dissolvingagent e is supplied from the sludge-dissolving agent storage vessel 15to the phosphorus elution acceleration vessel 27B. The mixer 19 isoperated to mix the residual liquid h and the sludge-dissolving agent ewith the defoaming blade 23 and the liquid mixing blade 21. Thedefoaming blade 23 may be removed if bubbles do not generate throughmixing or the like. A slight amount of phosphorus from a small volume ofthe sludge in the residual liquid h elutes into the liquid phase. Thephosphorus eluted sludge-containing liquid b is discharged to theoutside of the vessel through the discharge pipe 17D. The liquid levelin the phosphorus elution acceleration vessel 27B can be adjusted to aconstant height by providing an overflow type discharge pipe 17D or byproviding a liquid level sensor in the vessel to discharge thephosphorus eluted sludge-containing liquid b using a pump interlockedwith the sensor. Any ozone gas which could not dissolve in thesludge-containing liquid a is decomposed into oxygen and discharged tothe atmosphere by the waste ozone gas decomposition device 16 connectedto the upper portion of the phosphorus elution acceleration vessel 27B.

In Embodiment 5 of the present invention, most of the sludge is adsorbedon the bubbles in the ozone treatment vessel 11A, and thus, the volumeof the sludge remaining in the residual liquid h is small and theresidual liquid h contains only about several % to several tens % of SSin the sludge-containing liquid a. Thus, the sludge-dissolving agent emay be added and mixed into the phosphorus elution vessel 12C, tothereby elute phosphorus. On the other hand, the sludge-dissolving agente need not be added to the phosphorus elution acceleration vessel 27B,to thereby return the residual liquid h to a water treatment system asit is. Thus, sludge treatment volume may be reduced.

The residence time of the phosphorus eluted sludge-containing liquid bin the phosphorus elution acceleration vessel 27B is preferably 30minutes or less. A residence time exceeding 30 minutes requires a largephosphorus elution acceleration vessel 27B, which may lead to a largesludge treatment apparatus 32.

According to Embodiment 5 of the present invention, thesludge-dissolving agent e can be added depending on the phosphoruscontent in the bubbles c with the sludge adsorbed and in the residualliquid h. That is, the amount of the sludge-dissolving agent e added canbe adjusted depending on the volume of the sludge, the phosphoruscontent, and the like, thereby effectively maintaining a high phosphoruselution rate.

In Embodiment 5 of the present invention, the method for supplying thesludge-dissolving agent e is not particularly limited. Thesludge-dissolving agent e may be supplied as a shower from the upperportion of both the phosphorus elution vessel 12C and the phosphoruselution acceleration vessel 27B. Alternatively, the sludge-dissolvingagent e may be directly supplied just before the sludge adsorbed bubblesc and the residual liquid h are supplied or the phosphorus elutedsludge-containing liquid b is supplied to the phosphorus elution vessel12C and the phosphorus elution acceleration vessel 27B.

Embodiment 6

FIG. 17 is a diagram illustrating a sludge treatment apparatus 33according to Embodiment 6 of the present invention.

In FIG. 17, the sludge treatment apparatus 33 is provided with an ozonetreatment vessel 11, a phosphorus elution vessel 12A, a phosphoruselution acceleration vessel 27, a phosphorus eluate separating vessel34, a coagulation reaction vessel 35, and a phosphorus compoundseparating vessel 36. The phosphorus elution vessel 12A is connecteddownstream of the ozone treatment vessel 11, and the phosphorus eluateseparating vessel 34 is connected downstream of the phosphorus elutionacceleration vessel 27. The coagulation reaction vessel 35 is connecteddownstream of the phosphorus eluate separating vessel 34, and thephosphorus compound separating vessel 36 is connected downstream of thecoagulation reaction vessel 35. A residual sludge drawing pump 37 isconnected to a bottom portion of the phosphorus eluate separating vessel34, and a coagulant storage vessel 39 which temporarily stores acoagulant is connected to the coagulation reaction vessel 35 through acoagulant supply pump 38. A phosphorus compound drawing pump 40 isconnected to a bottom portion of the phosphorus compound separatingvessel 36. The construction up to the phosphorus elution accelerationvessel 27 is the same as the construction shown in FIG. 6, and thus, thesame portions as those in FIG. 6 are represented by the same reference,numerals and descriptions thereof are omitted in Embodiment 6 of thepresent invention.

Sludge treatment using such a sludge treatment apparatus 33 involves:supplying a phosphorus eluted sludge-containing liquid b from thephosphorus elution acceleration vessel 27 to the phosphorus eluateseparating vessel 34 through a discharge pipe 17B; and separating thephosphorus eluted sludge-containing liquid b into a phosphorus eluatecontaining a large amount of phosphorus eluted from the sludge andresidual sludge f. Examples of available separation methods includeprecipitation, centrifugation, floatation, and membrane separation.

The phosphorus eluate separated in the phosphorus eluate separatingvessel 34 is supplied to the coagulation reaction vessel 35, while thecoagulant is supplied from the coagulant storage vessel 39 to thecoagulation reaction vessel 35 through the coagulant supply pump 38.Carbonate groups are removed through previous treatment, and thus,phosphorus in the phosphorus eluate reacts efficiently with thecoagulant, resulting in a large amount of a coagulated phosphorouscompound g. The phosphorus eluate treated in the coagulation reactionvessel 35 is supplied to the phosphorus compound separating vessel 36and is separated into the coagulated phosphorus compound g (solid) andsupernatant water i. The phosphorus compound g is drawn with thephosphorus compound drawing pump 40 and is recovered. The supernatantwater i contains a large amount of organic substances, and can be usedfor energy recovery such as organic acid fermentation or methanefermentation.

Meanwhile, the residual sludge f separated in the phosphorus eluateseparating vessel 34 is drawn with the residual sludge drawing pump 37and is discharged to the outside of the vessel. The residual sludge f ismodified to be easily biodegradable, and is concentrated duringseparation of the phosphorus eluate and the residual sludge f. Thus, theresidual sludge f can be used for energy recovery such as organic acidfermentation or methane fermentation. For example, valuable substancesexcept phosphorus (such as protein, magnesium, and potassium) or energycan be recovered by: supplying the residual sludge f drawn with theresidual sludge drawing pump 37 to an adjustment vessel; adjusting pH orthe like of the residual sludge f; and supplying the residual sludge fto an anaerobic digestion vessel for acid fermentation or methanefermentation.

The coagulation reaction vessel 35 only needs to be a vessel that allowsan efficient coagulation reaction between phosphorus and the coagulant,and is preferably provided with a high speed mixer or a static mixer.

Further, examples of the coagulant include calcium chloride, hydratedlime, quick lime, ferric chloride, aluminum sulfate, polyacrylamide(PAC), a polymer coagulant, zirconium oxide, and magnesium. When analkaline aqueous solution is particularly used as a sludge-dissolvingagent e (pH of sludge of 9 or more), a calcium-based coagulant is usedas the coagulant, to thereby precipitate the phosphorus compound geasily. The supply of the calcium-based coagulant in this case ispreferably 2 to 10, more preferably 3 to 9 in molar ratio (Ca/P ratio)with respect to phosphorus in the sludge-containing liquid.

According to Embodiment 6, the carbonate groups in the phosphorus eluateare removed, allowing efficient recovery of phosphorus and sludge volumereduction.

Effects similar to those in Embodiment 6 may be obtained by connectingthe upstream of the phosphorus eluate separating vessel 34 with thephosphorus elution vessels 12, 12A, 12B, and 12C and the phosphoruselution acceleration vessels 27, 27A, and 27B in other embodiments. Thatis, the construction upstream of the phosphorus eluate separating vessel34 may employ any of those in Embodiments 1 to 5 (FIGS. 1, 6, 9, 13, and14). Of those, when the construction of Embodiment 5 (FIG. 14) isemployed, the phosphorus eluted sludge-containing liquid b may besupplied respectively from the phosphorus elution vessel 12C and thephosphorus elution acceleration vessel 27B to the phosphorus eluateseparating vessel 34. Alternatively, the phosphorus elutedsludge-containing liquids b from the phosphorus elution vessel 12C andfrom the phosphorus elution acceleration vessel 27B may be mixed onceand then supplied to the phosphorus eluate separating vessel 34.Further, only the sludge-containing liquid b from the phosphorus elutionvessel 12C need be supplied to the phosphorus eluate separating vessel34, and the sludge-containing liquid b from the phosphorus elutionacceleration vessel 27B may be supplied to other treatment systems suchas a water treatment system and a sludge treatment system. Thus, sludgetreatment volume may be reduced.

In Embodiment 6 of the present invention, a coagulation separationmethod was employed as a phosphorus recovery method, but the phosphorusrecovery method is not limited thereto. Phosphorus recovery methods suchas crystallization methods, magnesium ammonium phosphate (MAP) methods,and adsorption methods can also be employed.

In the sludge treatment apparatuses of the present invention, the upperportions of the ozone treatment vessels 11 and 11A are hemispherical butare not limited to those as long as the shape allows smooth delivery ofthe bubbles c with the sludge adsorbed through the bubble discharge port22. Further, the ozone treatment vessels 11 and 11A may be operatedwhile the volume of bubbles are controlled by automatically controllingthe number of revolutions or position of the bubble mixing blade 20, ormay be operated without the bubble mixing blade.

In the sludge treatment apparatuses of the present invention, the wasteozone gas decomposition device 16 is provided above both the phosphoruselution vessels 12, 12A, 12B, and 12C and the phosphorus elutionacceleration vessels 27, 27A, and 27B, but may be provided above onlythe phosphorus elution vessels 12, 12A, 12B, and 12C.

In the sludge treatment apparatuses of the present invention, a bottomcover may be provided at the bottom portions of the phosphorus elutionvessels 12, 12A, and 12B. The bottom cover may be controlled to open andclose by interlocking with an amount of a mixed liquid accumulated atthe bottom portions of the phosphorus elution vessels 12, 12A, and 12Bor controlled to open and close through timer control. The control ofthe bottom cover can facilitate operation management of the sludgetreatment apparatuses 10, 26, 29, 30, and 33. Further, the phosphoruselution vessels 12, 12A, 12B, and 12C may rotate to eliminate bubblesthrough centrifugal force. Thus, the bubbles are eliminated in a shortperiod of time, thereby accelerating reaction between the sludge and thesludge-dissolving agent e.

1. A sludge treatment method, comprising the steps of: foaming asludge-containing liquid by blowing an ozone-containing gas into thesludge-containing liquid to generate sludge-adsorbed bubbles anddischarge the bubbles from a bubble discharge port provided in an upperportion of an ozone treatment vessel, wherein the ozone-containing gasis blown into the sludge-containing liquid during the foaming step usinga diffuser pipe provided at the bottom of the ozone treatment vessel,and wherein the foaming step further includes mixing thesludge-containing liquid and the bubbles using a bubble mixing bladeprovided at an upper portion of the ozone treatment vessel and a liquidmixing blade provided at a lower portion of the ozone treatment vessel;eluting phosphorus in the sludge-containing liquid by supplying thebubbles to a phosphorous elution vessel from the bubble discharge portthereby bringing sludge adsorbed on the bubbles and a sludge-dissolvingagent into contact with each other within the phosphorous elutionvessel; and decomposing waste ozone gas not dissolved in thesludge-containing liquid into oxygen, wherein the waste ozone gas isreceived from an upper portion of the phosphorous elution vessel.
 2. Asludge treatment method according to claim 1, further comprising thestep of: precipitating a phosphorus compound by separating thephosphorus eluted sludge-containing liquid into a phosphorus eluate anda residual sludge and by adding a coagulant to the separated phosphoruseluate.
 3. A sludge treatment method according to claim 1, furthercomprising the step of: discharging the oxygen into the atmosphere.
 4. Asludge treatment method according to claim 1, further comprising thestep of: accelerating phosphorus elution by mixing the phosphorouseluted sludge-containing liquid.
 5. A sludge treatment method accordingto claim 4, further comprising the step of: circulating sludge betweenthe phosphorous elution vessel and a phosphorous elution accelerationvessel within which the accelerating step is performed.
 6. A sludgetreatment method according to claim 4, further comprising the step of:precipitating a phosphorus compound by separating the phosphorus elutedsludge-containing liquid into a phosphorus eluate and a residual sludgeand by adding a coagulant to the separated phosphorus eluate.
 7. Asludge treatment method according to claim 1, further comprising thesteps of: receiving residual liquid from the foaming step that was notadsorbed on the bubbles; and accelerating phosphorus elution by mixingthe phosphorous eluted sludge-containing liquid and the residual liquid.8. A sludge treatment method according to claim 7, further comprisingthe step of: circulating sludge between the phosphorous elution vesseland a phosphorous elution acceleration vessel within which theaccelerating step is performed.
 9. A sludge treatment method accordingto claim 7, further comprising the step of: precipitating a phosphoruscompound by separating the phosphorus eluted sludge-containing liquidinto a phosphorus eluate and a residual sludge and by adding a coagulantto the separated phosphorus eluate.
 10. A sludge treatment methodaccording to claim 1, further comprising the steps of: receivingresidual liquid from the foaming step that was not adsorbed on thebubbles; and accelerating phosphorus elution by bringing the residualliquid and a sludge-dissolving agent into contact with each other.
 11. Asludge treatment method according to claim 10, further comprising thestep of: circulating sludge between the phosphorous elution vessel and aphosphorous elution acceleration vessel within which the acceleratingstep is performed.
 12. A sludge treatment method according to claim 1,wherein the eluting phosphorous step further includes mixing phosphoruseluted sludge-containing liquid within the phosphorous elution vessel.13. A sludge treatment method according to claim 12, wherein the mixingis performed using at least one liquid mixing blade provided at a lowerportion of the phosphorous elution vessel and at least one defoamingblade provided above the at least one liquid mixing blade.
 14. A sludgetreatment method according to claim 13, wherein a greater number ofdefoaming blades are provided in the phosphorous elution vessel thanliquid mixing blades.